2D Fire Effect in ActionScript 3

I had a little bit of time today to do some me-coding (as opposed to work-coding), so I knocked up a quick 90’s era 2D fire effect. I’d actually written it C++ and OpenGL the other week and was meaning to transfer it into WebGL so it can run live rather than having a captured video, but my WebGL kung-fu is pretty weak at the moment so just to get it done I translated it to Flash.

How it works

The effect itself is incredibly simple, as the video below explains. You randomly add “hot-spots” to the bottom of the pixel array, then the new temperature value for a pixel is just the average of the pixel and the 3 pixels below it with a small amount subtracted so that the flames “cool”, which you then map to a colour gradient.

Sneaky!

Flash implementation

In the OpenGL version I’d made the colours for the flames into a 1D texture which can be easily interpolated, but I had to find some functions to do that in AS3 as there don’t appear to be 1D textures! Also, getting and converting the colours from uints and hex-codes and stuff was a pain – but I finally nailed it after googling around and pulling functions from various places (referenced in the source).

I think the most important thing I learnt from getting this working in Flash was how to and how NOT to convert colours from uints into red/green/blue components. For example, a lot of code online will use a function like the following to convert red/green/blue values into a 24-bit uint:

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// NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!!

functionrgbToUint(red:uint,green:uint,blue:uint):uint

{

varhex="0x"+red.toString(16)+green.toString(16)+blue.toString(16);

returnhex;

}

The above function looks like it works, and in fact in most cases does work – but if you get single digit values they aren’t zero-padded properly and craziness ensues!

The correct way to convert RGB to uint is by using the two functions below which ensure that values are capped AND zero-padded as appropriate, which means that everything works under all possible conditions:

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functionconvertChannelToHexStr(hex:uint):String

{

if(hex>255)

{

hex=255;

//trace("hex val overloaded");

}

varhexStr:String=hex.toString(16);

if(hexStr.length<2)

{

hexStr="0"+hexStr;

}

returnhexStr;

}

functiongetHexColourFromRGB(r:uint,g:uint,b:uint):uint

{

varhexStr:String=convertChannelToHexStr(r);

hexStr+=convertChannelToHexStr(g);

hexStr+=convertChannelToHexStr(b);

varstrLength=hexStr.length;

if(strLength<6)

{

for(varj:uint;j<(6-strLength);j++)

{

hexStr+="0";

}

}

varfinalStr="0x"+hexStr;

returnfinalStr;

}

Wrap up

Overall, it’s a nice simple effect – but it’s pretty CPU intensive. In the above example I’m only using a stage size of 250px by 120px and the framerate takes a hit even then.

I could use a single loop instead of embedded x and y loops, and I could manipulate more colours directly as uints rather than pulling out the RGB components, but it’s prolly not going to improve more than about 10-15%. As usual, there’s source code after the jump, so if you have a play with it and manage to speed it up a bunch, feel free to let me know how you did it!

Cheers!

Download Link: 2D-Fire-Effect.fla (Flash CS4 fla – but CS5 and later will convert on load).

Source Code

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import flash.display.BitmapData;

// Set up BitmapData instances the size of the screen, no transparency and filled with black

3 thoughts on “2D Fire Effect in ActionScript 3”

I think a way of improving performance on this code is to try and remove the conditional ‘nest’. I’ve noticed that removing conditionals and simplifying code can speed things up, however doing this will make tweaking things much harder. So, swings/roundabouts and all that

Also having these functions may save you time in the future.
You could generalise it by having source be an int or a String or whatever. Don’t know if this language is OO’esque, but if so then that would be simple.